In the automotive industry, due to the demand for improved safety and reduced fuel consumption, the need to manufacture lightweight body parts by hot stamping is rapidly increasing. Although hot stamped parts provide tensile strength above 1.5GPa by martensitic microstructure, it has deficient crashworthiness because of low ductility. Therefore, recent research of hot stamping has been tried to manufacture multi-strength parts having both high and low strength region by advanced process such as post-tempering of hot stamped parts, quenching with partially heated tools or hot stamping using partially heated blank.
In concept of hot stamping using partially heated blank, the blank is differentially heated to control local temperature of the blank below Ac3 in which heating condition causes to retain ferritic-pearlitic microstructure. Hence, the temperature difference on the blank directly affects the temperature gradient, cooling rates and microstructure which leads to the mechanical property of hot stamped parts.
This paper aims to predict the hardness of multi-strength hot stamped parts using partially heated blank by the FE-simulation coupled with quench factor analysis(QFA) and Johnson-Mehl-Avrami-Kolmogorov(JMAK) equation. First of all, dilatometer test of boron steel was performed using the dilatometer with forced air cooling system. The dilatometer test provided a hardness data according to cooling rates which were used to determine the material constants of QFA and the time-temperature-property(TTP) diagram of boron steel. The measurement of phase fraction was also performed to consider transformation characteristics of austenite depending on heating conditions such as heating temperature and time. This measurement provided a phase fraction of austenitic structure according to heating histories which were used to determine the material constants of JMAK equation. Then, FE-simulation was performed to predict the heating and cooling histories for each stages in hot stamping such as heating, transferring, forming and quenching in which predicted results were used to calculate the hardness of multi-strength hot stamped parts. Furthermore, experiment of hot stamping using partially heated blank was performed to verify the predicted hardness resulted from FE-simulation coupled with QFA and JMAK equation. Finally, predicted results were compared with those of experiment which shown that the predicted hardness was in good agreement with experimentally measured one.